This invention relates to test fixtures for the automatic testing of loaded printed circuit boards, and more particularly to a test fixture utilizing a tilt pin translating fixture that contains an optimized translation for the test probes for testing high pitch density test patterns.
Automatic test equipment for checking printed circuit boards has long involved the use of a “bed-of-nails” test fixture to which the circuit board is mounted during testing. This test fixture includes a large number of nail-like, spring loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test (UUT). Any particular circuit laid out on a printed circuit board (PCB) is likely to be different from other circuits, and consequently, the bed-of-nails arrangement for contacting test points in a particular circuit board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in testing is selected and a corresponding array of test probes is configured in the test fixture. This method typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes in the probe plate. The circuit board is then mounted in the fixture, superimposed on the array of test probes. During testing, the spring loaded test probes are brought into spring pressure contact with the test points on the UUT. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points and the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the PCB under test into pressure contact for testing loaded printed circuit boards. One class of these fixtures is a wired test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures were dedicated fixtures and are often referred to as “vacuum test fixtures” since a vacuum may be applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made by using mechanical means, other than vacuum, to apply the spring force necessary for compressing the board into contact with the probes during testing.
Another class of test fixtures for testing bare printed circuit boards is the so-called grid type fixture in which the test points on the test side of a board are contacted by flexible pins or tilt pins which can move or otherwise be positioned to contact the random pattern of test points on the board and transfer test signals from the board to sets of interface pins arranged in a grid pattern on the receiver. In these grid type testers, fixturing is generally less complex and simpler than the customized wired test fixtures because there is no need to individually hard wire the test probes to separate interface contacts for each differently configured circuit to be tested; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly.
In a grid type fixture for bare circuit board testing, the wiring for the grid array remains constant, independent of the configuration of the particular circuit board. What does change, however, is what is referred to as the translator fixture. The translator fixture includes a bottom plate having a hole pattern corresponding to the grid pattern of openings in a standard pin grid array, and a top plate having a hole pattern corresponding to the random off-grid pattern of contact points to be tested on a printed circuit board. A number of electrically conductive translator pins (these can be flexible pins or rigid tilt pins) are mounted in the holes of the top and bottom plates. As the translator pins travel through the translator fixture, they are redirected by the hole patterns of the plates to provide individual conductive paths between the standard grid pattern and the off-grid pattern corresponding to the test points on the circuit board under test. Extreme contact accuracy can be achieved between the translator pin and the test pad on the PCB because the translator pin does not extend beyond the upper surface of the top plate. The top plate in effect can accurately direct the translator pin precisely to the test pad through the holes in the top plate. The construction of the grid type translator fixture is typically less labor intensive than the rewiring of test probes in a wired type test fixture, making it simpler to customize the fixture to accommodate PCB's with different test point patterns. Therefore, it is often desirable to use a grid type test fixture when testing printed circuit boards having various different shapes and/or configurations.
The present invention is directed to test fixtures for testing loaded circuit boards. A problem encountered with these types of fixtures for testing loaded circuit boards is the ability to test high density test points on the printed circuit board. Many PCB tests utilize fine pitch test points arrangements, i.e., a spacing of 50 mils or less between test points. However, spring probes suitable for fine pitch spacing on the probe plate do not have a spring force sufficient to provide good electrical contact with the test points on the PCB. The present invention overcomes this problem by translating a higher density probe arrangement at the interface between the test probes and the PCB under test into a wider-space probe arrangement, typically 100 mils on center, in the test fixture. Having a test probe pattern on the probe plate with a sufficiently wide spacing permits the use of heavy duty spring probes, commonly referred to as 100 mil spring probes, which deliver a greater spring force, providing better electrical contact. This force is translated through tilt pins, providing better electrical contact with a fine pitch test point arrangement on the PCB under test. One such loaded board test fixture is disclosed in applicant's U.S. Pat. No. 5,818,248, the disclosure of which is incorporated herein by reference.
However, a need exists for improvements in optimized translation of the tilt pins of the loaded board test fixture disclosed in the '248 patent, as well as the ability to test individual components of the UUT.